In a head-up display (HUD), optical design is made more complex by a design constraint to keep the optical relay lens or lenses out of the direct line of sight of the driver, pilot or viewer. Typically, meeting this design constraint results in adding a combiner or using the windshield as a combiner where the function of the combiner is to combine the image from the HUD projector with the view of the world directly in front of the driver, pilot, or viewer. The main relay optics for an automotive HUD are typically housed inside the dashboard, with the output light projected upward onto the windshield which reflects a portion of the light back towards the driver where not much space may be available, and where the optical path may be relatively complex.
It will be appreciated that for simplicity and/or clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, if considered appropriate, reference numerals have been repeated among the figures to indicate corresponding and/or analogous elements.